1. Field of the Invention
This invention relates generally to a data cable and, more particularly, to a data cable that dissipates the static charge that is produced on the surface of the data cable as a result of dry air blowing across its outer surface.
2. Description of Prior Art
A data cable used in an outdoor environment (such as those found on seismic surveys) is subject to many harsh conditions. The cable typically must endure temperature, humidity, and ultra-violet radiation extremes that range from the desert, to the rain forest, to the Arctic. The jacket of the cable must be abrasion resistant to withstand the rough handling on the cable that may be retrieved and deployed several times a day over all types of terrain in all ranges of climates. To successfully endure these conditions, traditional cables have used high-quality outer jacketing materials such as polyurethane. In addition to the properties necessary to survive adverse listed conditions, these materials typically are moderately good electrical insulators, with bulk volume resistivity values of about 10.sup.12 ohm-cm.
As is well known, a static electrical potential difference is produced between dissimilar materials when they are rubbed together. This effectively occurs when (dry, perhaps sand-laden) air blows across the surface of an insulative cable jacket, due to the accumulation of electrical charge. If there is an effective path or means for the static charge to dissipate then no harm is done to the signal being carried by the conductors in the cable. However, if there is no such effective means present that allows dissipation of the charge as rapidly as it is accumulated then eventually voltage breakdown will occur between the outer jacket of the cable and the surrounding ground plane thereby creating a path through which the charge dissipates. When there is such a voltage breakdown, typically this results in a relatively high current arc, which is capacitively coupled as noise into the signals being carried by the conductors. The effectiveness of the path of dissipation is inversely proportional to the resistance values of the materials used to form the jacket of the cable; an insulator will retain a charge while a conductor will dissipate it.
Therefore, to reduce noise due to the build up of static charge, there exists a need to provide a cable wherein a better electrical path from the surface of a cable (where the charge accumulates) to the ground on which it is lying. For outdoor applications such as seismic surveying, this problem is typically worse on very dry, windy days and almost non-existent on rainy days. In severe cases in seismic applications, prior art has suggested a labor intensive method of wetting the cable and/or the nearby surrounding soil, which in remote and desert areas may be impractical but in any event since seismic surveying is conducted over a vast area at a time such a solution can be very expensive. Furthermore, this solution provides only temporary relief, i.e., until the moisture has been evaporated.
Thus, there remains a need of providing a data cable which effectively dissipates or discharges static charge built up on the surface of the cable jacket.